Take photos as you go so you can see what went where or, more importantly, which way round.
When I ran my shop I would see the neglected shotguns start coming in around the end of February; most had been put away at the end of the previous duck shooting season and when they were pulled out of their gun bags they always looked a little worse for wear. The biggest issues were corrosion, and in the case of semi-autos some were gummed up with a combination of corrosion and powder fouling. So if you have just taken your pride and joy out of the safe and it’s looking a bit sad, it could be time for a bit of love.
But before you start working on your gun, check that it’s unloaded - and look into the chamber/s and magazine tube to be 100% sure.
Corrosion or rust is one of the biggest enemies of any firearm; put it away wet at the end of the season and you are likely to have some ferrous oxide forming - and if it’s on the barrel then it’s likely it’s also hiding under the woodwork, so strip the shotgun as far as you can.
If you are uncomfortable with this type of work, you are likely to find a video somewhere on the internet showing you how it’s done, but if you can’t find anything to help you then make sure you have a tray ready for the small parts, as losing them can be a real pain.
Take photos as you go so you can see what went where or, more importantly, which way round. When you have the gun stripped, inspect it and give it a good wipe down to remove the old lubrication. If there is rust you should remove it. If it’s light to medium, apply some oil onto the affected area. Wait for a few hours for it to penetrate and polish it off with steel wool; lubricated steel wool will remove the rust while leaving the blued surfaces intact. Never use sandpaper or scrape with a steel instrument; this leaves ugly scratches that look as bad as the rust.
Once it’s removed, inspect it and make sure there is no deep pitting. If the pitting from the corrosion is on the magazine tube of a semi auto it may impede the operation of the gas system, so try it before opening day to make sure it functions.
Now that you have removed the demon rust you need to apply a coating to stop it coming back. If you’re out on a big weekend and it’s wet, give your gun a wipe-down and reapply your gun oil; a light wipe is all that’s necessary. Alloy receivers and synthetic stocks don’t need it - only the steel.
Removing the carbon from the gas system of semi autos is straightforward: it can be wiped or brushed off in some cases but the application of some carbon-specific bore cleaner will likely make this a lot easier. I also like to use super penetrating oils like Kroil, but these can take a few hours or more to get underneath the carbon and loosen it. Products like Slip 2000 Carbon Killer, Hoppes no 9, Boretech C4, Breakfree Carbon Cutter and KG1 are also very good at removing carbon.
In some cases it’s necessary to polish the last bits of carbon fouling off with steel wool; I use the same process as you would for removing corrosion: some penetrating oil and then polish off.
Give the barrel a good scrub. Use a rod and an appropriately-sized jag, get plenty of cleaning fluid onto the bore surface, and let it get to work. Unlike rifle barrels there is no copper fouling and in most cases there is no lead fouling either - but there can be plastic wad fouling, and there are specific shotgun blends of cleaning fluid that will attack and remove this. Use a good stiff brush and keep going until it’s all out. With tough wad fouling you may have to let the cleaning fluid penetrate overnight then start again the next day. In a worst case scenario use some JB bore paste and polish it out.
It pays to pull the trigger unit out, and on most modern pump action and semi-auto guns this means removing 1 or 2 pins. First, cock the gun then let the bolt go forward; you can then tap the pin/s out and remove the trigger unit. Brush any fouling or debris away with a toothbrush, and give the moving parts a drop of oil. Reassemble in the reverse order.
To remove the bolt you often have to take out the cocking handle - in many cases this comes out with a strong pull. The bolt on a pump will be attached to the action bars and these can be accessed when the trigger unit is removed. Check the bolt; most bolts are either rotating or they have a locking lug that comes up into the receiver of the barrel extension. There is often a pin at the rear of the bolt that will allow the firing pin to come out; remove this and in rotating bolt guns this will usually allow the bolt-retain key to come out and then the bolt head can be removed. Wipe the surfaces down, check for corrosion, add a few drops of lube and reassemble.
Thanks to Wayne at the Upper Hutt Gun Shop for letting me pull apart all his shotguns; luckily he had a good selection available.